The indecent incident occurred last Saturday, August 12 on the 7.33pm ScotRail service from Edinburgh Waverley to Dunblane.
This took place as the train passed between Linlithgow and Falkirk Grahamston.
Police said the woman felt embarrassed and violated by the man’s behaviour.
The man, believed to be in his forties, is white, around 5ft 8ins and had fair receding hair. He was wearing a black Adidas t-shirt and sandy shorts.
He was travelling with two teenage boys.
Anyone who witnessed the incident or recognises someone who matches this description is urged to get in touch with British Transport Police by calling 0800 40 50 40 or texting 61016.
Information can also be given anonymously to Crimestoppers on 0800 555 111.